Liquid developers for electrostatic photography



June 2, 1964 w. c. YORK 3,135,695

LIQUID DEVELOPERS FOR ELEC'IROSTA'JSIC PHOTOGRAPHY Filed Feb. 20, 1961Pigment Particle Encupsulufing aromatic soluble resin envelope DiffuseNegative Layer in Currier Liquid WILL/AM 6. YORK 1N VEN TOR.

A TTOR/VEYS United States Patent 3,135,695 LIQUE DEVELQPERS FQRELECTRGSTATIC PHOTOGRAPHY William C. Yorlr, Rochester, Nfl Z, assignorto Eastman Kodak Company, Rochester, N.Y., a corporation of New JerseyFiled Feb. 29, 1961, Ser. No. 9%,494 3 Claims. (Cl. 252-62.l)

This invention relates to Xerography and more par ticularly to a novelcomposition of matter for developing a visible image from a latentelectrostatic image existing as a charged pattern on xerographic plates.

Processes for producing a visible image from a latent electrostaticimage by developing with an electrostatically attractable dry powderhaving optical density are well known and have been described innurnerous US. patents including the Carlson Patents 2,297,691, issuedOctober 6, 1942, and 2,357,809, issued September 12, 1944. To develop alatent electrostatic image, the dry granular electroscopic powders havein the past ordinarily been cascaded over latent electroscopic images orbrought in the form of powder clouds into intimate contact with thecharge image to be rendered visible. Dry developers of tlu's type havebeen described in such US. patents as Wise 2,618,- 552, issued November18, 1952, and Landigran 2,753,308, issued July 3, 1956. More recently ithas been proposed to develop latent electrostatic images using liquidmedia rather than the above-described dry processes. Straughan in [1.8.2,899,335, issued August 11, 1959, for example, proposed the developmentof an electrostatic image using a liquid developer in which the chargedsolid developing particles are carried into contact with theelectrostatic image in a liquid medium instead of in air or a gaseousmedium as in the aforesaid Carlson patents, for example. Another type ofcharged pigment-suspending liquid developer of electrostatic images hasbeen described by Metcalfe and Wright in Australian Patent 212,315,patented December 6, 1956. Liquid xerographic developers for the mostpart, however, have encountered a difiiculty in stability in that theytend to clump on standing and in many cases the charged pigment particletends to lose charge, exhibits a charge reversal, or a change inpigment-tobinder ratio, etc. The present invention overcomes, veryunexpectedly, each of these persistent dir culties by a relativelysimple process such that charged pigment-suspending liquid developersare stable beyond 800 hours.

It is, therefore, a primary objective of the present in vention toprovide liquid electrostatic developers which have improved stability.Another object of the present invention will be to provide a process forpreparing the stable liquid electrostatic developers of the presentinvention. A further object is to provide a process for developing anelectrostatic image to a visible image using an electrostatic developerof the present invention. ther objects will become apparent from areading of the specification and appended claims.

The objects of the present invention are accomplished by surroundingindividual xerographic type pigment particles with a suitableresinous-binder envelope, treating the pigment-binder combination with asmall amount of an alkyl-aryl compound before suspending the combinationin a liquid aliphatic carrier. The resin used to enclose the pigmentparticles of the present invention can be any of the well known resinswhich are soluble in aromatic solvents, but insoluble, or at least onlyslightly soluble, in aliphatic solvents. The liquid aliphatic carrieraccording to the present invention must have a low dielectric constantand a resistivity of at least about ohm centimeters to preclude thepossibility of discharging the latent electrostatic image. The lowdielectric constant is needed so that the energy in the image field isnot dissipated in aligning and moving highly polar molecules. If thepigment-binder combination, i.e., toner, of the present invention isdispersed in the liquid aliphatic carrier without first treating it withan alkyl-aryl compound, the system coagulates badly. Where thepigment-binder combination is prepared, treated and suspended in analiphatic carrier according to the present-invention, liquidelectrostatic developers have been prepared which are stable beyond 800hours without apparent deleterious efiect. While I do not wish to bebound by any specific theory of operation, I believe that the alkyl-arylcompound used as a treating means for the pigment-binder combinationbecomes oriented so that the aryl group is directed inward towards thearomatic soluble binder and the aliphatic tail is directed outwardforming a liquid bridge between the aromatic soluble binder and thealiphatic carrier. In any event the liquid electrostatic developers ofthe present invention remain stable over very long periods with thebinder remaining as a discrete envelope surrounding the pi ment particleand without the binder diffusing into the carrier liquid. Where suchstability of the binder in liquid developers of this type is notobtained there results a number of deleterious effects including anundesirable shift in the pigment to binder ratio with progressive useand age of the developer.

11 the single figure of the accompany ng drawing is shown a hypotheticalorientation of an allryl-aryl compound, propyl benzene, to the aromaticsoluble resin envelope surrounding a pigment particle having a positivecharge. The toner, i.e., pigment particle-resinous envelope combinationis, of course, according to the invention suspended in an aliphaticliquid carrier which is represented in the present drawing as ahypothetical diffuse negative layer surrounding the positively chargedtoner particle.

In carrying out the present invention an electrostatic latent image canbe produced by any of a number of procedures recognized in the art. Forexample, a sensitive element may be produced by coating a support with asuitable photoconductor, such as those described in the Carlson patentscited above, and imparting a uniform charge to said sensitive element byany of the known means such as corona discharge and by rubbing as alsodescribed in the Carlson patents. Finally, the charged sensitive elementis exposed to a pattern of light and shadow illumination whereby theilluminated areas are discharged and a charged image pattern remainswhich is developed to a visible image by flowing a developer of thepresent invention over the differentially charged surface. Theelectrostatically attractable particles carried in the insulating liquidof the developer solution are brought into contact with theelectrostatic image which results in the particles being deposited onthe charged areas of the sensitive element so that the charged areas areimmediately rendered visible because of the optical character istics ofthe toner particles. Because of the highly volatile nature of thealiphatic carriers of the developer composition, fast drying results anda visible, permanent image is rapidly obtained. In preparing the liquidelectrostatic developers of the present invention it is the usualprocedure to mix and grind together in a ball mill or other suitablecomminuting equipment a suitable pigment having optical density with asolution of a resinous material which is soluble in aromatic solventsbut insoluble or only slightly soluble in aliphatic solvents. Thepreferred resinous binders should also be non-chalking, should'dry to atough wear-resistant film and should have a refractive index conduciveto enhance color or transparency. On the other hand suitable resinousmaterims should have no adverse effect on particle charge or ondeveloper stability. Resinous materials which I have 'either negative orpositive.

found suitable for use in the present stabilized liquid developers canbe selected from a number of synthetic resins, modified resins, rosinesters, modified rosin esters, etc. Examples of suitable resins to whichI refer are given in Table 1 below. i

TABLE I Trade name Resin type Manufacturer Epon 100 Polyethylene ShellOil Company. Amberol 226 Phenol-formaldehyde Rohm and Haas C0.

rosin-modified. Amberol B/S-l Do. Amberol F-7 Do. Amberol F-77 Do.

modified. Amberol K12A Do. Amberol M88- Do. Amberol M93-- Do. Amberol800 Do. Do. Do. Do. Amberlac D-96 --do Do. Cellolyn 102 Modifiedpentaery- Hercules Powder Co.

thritol ester of rosin. Cellolyn 104 --do D0. Lewisol 2L Maleiealkyd-modi- Do.

fied rosin ester. Lewisol 28 clo D0. Neolyn 23 Rosin derived alkyd- Do.

type resin. Neolyn 35 -do Do. Pentalyn B25 Phenol modified Do.

pentaerythrit-ol ester of rosin. Pentalyn 802A do Do. Arochem 75Modified phenolic--.. Arzher Daniels Midland o. Arocbem 333 Do. Aroehem335..- Do. Aroehem 337. Do. Arocbem 339- Do. Arochem 355. Do. Arochem402 Do. Arochem 40 do Do. Arochem 520. Modified maleie Do. Aroeliem 533-Modified penta ester Do. Arochem 542- M dified maleic.- Do. Aroehem 545.Do. Aroehem 095. Do. Aroohem 650 cquer resin--- Do. Aroiene 753 Purephenolic resin-- Do. XLH 403.--- Modified pentaery- Sherwin-Williams Co.

thritol. XL 4 -do Do. Dymal XLM 110 aleie alkyd D0. Dyrnal XLM 101.. dDo. Dymal XLM 103.- Do. Dymal XLM 106-..- Do.

Other suitable resinous materials and mixtures of such materials havebeen described in such materials have been described in such US. patentsas Insalaco 2,891,011, June 16, 1959, and Carlson 2,940,934, June 14,1960.

The pigments which can be used in preparing the toners of the presentinvention can vary Widely. For example, in Carlson Patent 2,297,691,issued October 6, 1942, there are described numerous organic andinorganic materials, including talcum powder, aluminum bronze, carbondust, gum copal, gum sandarac, ordinary rosin, and others. The principalrequirements of any of the pigments used in the present invention isthat they are particulate and capable of carrying an electrostaticcharge. When finely divided pigments are suspended or dispersed in asuitable liquid vehicle, as is shown, the solid particles usuallyacquire a charge. With some pigments, however, it has been observed thata prepoderanw of the particles acquire a charge of one sign, while withother pigments, the number of positively-charged particles about equalsthe number of negatively-charged particles. For optimum efficiency, itis desirable to employ a pigment in which a preponderance of theparticles acquire a charge of one sign, In my investigation I havestudied a number of pigments Which I have found acquire substantially asign of one charge. Presented herein accordingly are a number ofpigments which have a negative particle charge in solution and a numberof pigments which acquire a positive particle charge in solution.Several of these pigments of each type are given in the table below. Thecarrier solution for determining the charge was cyclohexane.

Factor, Max & C0. General Dyestufi Co. Factor, Max dz Co.Sherwin-Williams Co.

Pyramide Cerise Toner RA 520 Fast Mono Green Toner G-FG 430.- PigmentYellow LX CyB-340 Solfast Green 63100 POSITIVE PARTICLE PIGMENTSAluminum Powder (flake) Antimony Sulfide Cuprie Sulfide--- Carbonyl IronLamp Black, Germantown Mapico Black Nigrosine S.S.B. No. 5.

General Aniline and Film Corp. Colombian Carbon 00.

Do. General Aniline and Film Corp.

Grassol Black Geigy 00., Inc. Monastral Fast Blue B E.CI. du Pont deNemours and o. Dresden Blue 81451 Imperial Paper and Color Corp. PyramidGermanium Toner RA 500 E Factor, Max 6: Co.

Selkirk Red 8.- Imperial Paper and Color Corp.

Winthrop Red X4666. Britone Red M CP974. Britone Red MCP1290 DuratintGreen 48-238. C.P. Light yellow X-179 W81 Benzidine yellow Anilide Do.Sherwin-Williams C0.

D0. Factor, Max dz Co. Imperial Paper and Color Corp. Sherwin-WilliamsCo.

With either the negative or positive pigment particles an alkyl-arylstabilized toner can be prepared by milling together a suitable pigmentwith a suitable solution of resin or mixture of such resins as set forthhereinabove. Suitable carriers for dispersing the stabilized tonerscomprise liquids possessing a'high dielectric resistance. In order toavoid discharge of a latent electrostatic image, a liquid with aresistivity of about at least 10 ohm centimeters is necessary. Liquidswhich I have found suitable include cyclohexane, carbon tetrachloride,mixed halides such as trichlorotrifluoroethane, sold under the tradename Freon 113 by E. I. du Pont de Nemours and Co., cyclopentane,n-pentane, n-hexane, etc. In preparing the stable developer compositionsof the present invention, heavier oils can be used, such as kerosene,mineral oil, silicone oil, etc. When these latter mentioned liquids areused, however, they usually require heat to set the toner and drive offexcess carrier.

According to the present invention the alkyl-aryl treating compoundemployed before dispersion of the pigmentbinder combination in asuitable carrier can be selected from the compounds represented by thefollowing general formula 'wherein R represents at least one straightchain or branched-chain alkyl group having from 1 to about 12 carbonatoms and Z represents the non-metallic atoms necessary to complete anaromatic nucleus selected from the class comprising benzene,naphthalene, and anthracene. Typical examples of solvents of the presenttype comprise xylene, mesitylene, ethylbenzene, n-propylbenzene,n-butylbenzene, isobutylbenzene, n-pentylbenzene, n-hexylbenzene,n-propyl naphthalene, butyl naphthalene, propyl anthracene, butylanthracene, and the like. Useful results were obtained using isomers aswell as the normal aliphatic hydrocarbons and mixtures of each thereof.The preferred alkyl-aryl compounds are those in which the arylsubstituent is benzene. Especially useful results were obtained wherethe aryl substituent is benzene and the alkyl group is selected fromthose having from 3 to 6 carbon atoms. Preparation of any of thealkyl-aryl compounds of this invention can be by any of the well knownmethods.

In preparing the stabilized liquid developer solutions of the presentinvention the toner is first prepared by mixing a uniform dispersion ofthe pigment in an aromatic solution of the resinous material andthereafter mixing and milling the combination to produce a finelydivided toner paste of desirable particle size. The mixing and millingmay be done by various means including a combination of the steps ofblending, mixing, and milling wherein the preferred method for themilling step includes a rather extensive milling period to assureuniform and fine dispersion or" the pigment in the resin. After milling,the toner is removed from the mill jar. The toner is then treated with asmall amount of an alkylaryl compound such as propylbenzene. Thetreatment required is usually dispersing the toner in the propylbenzeneand thoroughly mixing. After treatment, the dispersion is added to thealiphatic carrier of choice. Liquid electrostatic developers prepared inthis manner have been checked periodically over a period beyond 800hours without the developer showing signs of deleterious effects.

The following examples will serve to further illustrate practice of thepresent invention.

Example 1 Prior to addition in the above composition the Pentalyn M wasdigested in a 1:1 ratio in a hydrocarbon mixture sold commercially underthe trade name Solvesso 100 by the Esso Standard Oil Co.

After ball milling 0.5 gram of the toner was mixed with a compositioncomprising 0.5 ml. of n-propylbenzene and dispersed further in 190 ml.of cyclohexane. The resulting composition was flowed over a surfacebearing a latent electrostatic charge to quickly produce a clear visibleimage of good resolution. The image produced dried almost instantly to ahigh-smudge resistance. The developer of this example was testedperiodically beyond 800 hours with no sign of deterioration,agglomeration, change in binder-pigment ratio, etc.

xnmple 2 An alkyl-aryl stabilized liquid developer was used to developthe latent electrostatic image on xerographic printing material afterthermographic exposure according to the following procedure.

The xerographic printing material was prepared according to thedisclosure of Dulmage et al. U.S. application Serial No. 25,109, filedApril 27, 1960, and comprises coating a one-half mil thickness ofpolyethylene terephthalate taped onto a glass support with a 10 percentsolution of a lower melting polyester dissolved in methylene chloride.The coating was placed on the plate at a 0.006- inch wet thickness.After drying for about 3 minutes a sheet of paper 1.5 mil thickness wasrolled onto the tacky coating with a small hand roller and thecombination was oven-dried at 75 C. The polyethylene terephthalatesurface was then charged by passing under a negative corona wire at apotential of 10 kv. A document was laid onto the charged surface and thesandwich was given a reflex exposure under a high intensity infraredsource such as commonly used in thermography. After radiation exposurethe xerothermographic sheet was developed by allowing a liquid developerof the following composition to flow over the surface of the plate. Thetoner of negative charge adhered only to the areas from which theexcellent chemical resistance) 20 Amberol 808 (a trade name of Rohm andHaas Co.

for a modified maleic rosin) 20 The Amberol 808 was dissolved in hotSolvesso 100, a hydrocarbon solvent sold by the Esso Standard Oil Co.,in a 1:1 ratio before use in the above composition.

After ball-milling, 0.5 gram of the toner was dispersed in a liquidcomposition comprising 0.5 ml. isobutylbenbene and carbon tetrachloride.

The xerothermographic sheet developed with a negative toner of the abovecomposition was then placed in contact with a sheet of ordinarytypewriter paper and passed again under the infra-red lamp whichresulted in the image from the xerothermographic sheet being transferredto the typewriter paper to produce a permanent copy thereon. Excellentwell-defined images were obtained by the process of the present examplewith the images having a very clear background.

In preparing the stabilized electrostatic developers of the presentinvention it is usually required that only relatively small amounts ofthe alkyl-aryl stabilizing compound need be employed. Ordinarily thetoner particles are treated with an alkyl-aryl compound in an amount byweight about equivalent in a ratio of from about 1:5 to 5:1 toner toalkyl-aryl compound. The toner is usually contained in the finaldeveloper composition in concentrations in the range from about 1percent to about 20 percent by weight. Accordingly, the aliphaticcarrier can make up from about 60 percent to about 98 percent by weightof the total composition.

The wide range of resinous materials suitable for use in preparing thetoner particles according to the present invention makes possible a widerange of processes with which stabilized developers of the presentinvention can be used. One such process includes the making of colorprints. The instant-drying feature of the present invention with nosmearing and without requiring fusion of xerographic elements developedwith the liquid developers of this invention makes possible themulti-step processing necessary to achieve color prints. The fastdryinghighly volatile aliphatic carriers make possible an almost continuousprocess for preparing multi-color prints. By use of the small tonerparticles available for incorporation into a liquid developer accordingto the present invention makes it possible to obtain improved resolutionin the prints obtained. For example a resolution of about 40 lines permillimeter can be obtained on standard zinc oxide xerographic plateswhere a toner particle having a diametric size of about 1.0 micron isused. In processes of the present invention, various materials andcompositions thereof can be used to encompass a wide variety ofmaterials, for example, lithographic ink, gravure ink, silk screen ink,paints, artists colors, flash colors, etc. Moreover, liquid xerographicdevelopment can be used in preparing line copy, half-tone negatives orpositive, continuous tone prints, etc.

In preparing the toner particles of the liquid electrostatic developersof the present invention the particle size of the pigment and theelectroscopic toner should be in the range from about .1 micron to about20 microns. Particle sizes in this range when used with standardxerographic printing plates gave the desired degree of resolution. Theparticle size of the toners is usually the result of the time andmethods used in the ball milling process. For example, the particle sizeof many of the pigments can be reduced to less than 20 microns bymilling for about 6 to 8 hours. For purposes where the particle sizeshould be 10 microns or less an extended ball-milling period was usuallysatisfactory to produce the desired particle size. Where continuous tonecopy was to be developed using liquid developers it was usu ally desiredfor obtaining optimum resolution that particle sizes be reduced to about1 micron. In this case a super ball mill was required.

The invention has been described in detail with par-' ticular referenceto preferred embodiments thereof, but it will be understood thatvariations and modifications canbeeifected within the scope and spiritof the invention as described hereinabove and as defined in the appendedclaims.

I claim:

l. A stabilized liquid developer for developing latent electrostaticimages, said developer consisting essentially- 8 wherein R represents atleast one aliphatic- -group having from 1 to 12 carbon atoms, and thedispersion of toner in alkyl-aryl solvent being further dispersed in 4(3) a liquid carrier phatic hydrocarbon having an electrical resistivityof at least 10 ohm centimeters and a low dielectric constant, saiddeveloper containing, said toner and said alkyl-aryl solvent in a rangeof ratios by weight from about 1:5 to about 5:1, said toner making upfrom about 1 percent to about 20 percent of said developer and saidliquid carrier making up from about percent to about 98 percent byWeight of the said developer. 2. The stabilized liquid developer ofclaim 1 for developing latent electrostatic images in which the saidalkyl -aryl solvent is propylbenzene. v r

3. The stabilized liquid deveolper of claim 1 for developing latentelectrostatic images in which the resinous binder is a phenol modifiedpentaerythritol ester of rosin acid and the aliphatic hydrocarboncarrier is cyclohexane.

References Cited in the file of this patent UNITED STATES PATENTS2,877,133 Mayer Mar. 10, 1959 2,891,911 Mayer et al June 23, 19592,899,335 Straughan Aug. 11,1959 2,907,674 Metcalfe et a1. Oct. 6, 1959consisting essentially of an ali

1. A STABILIZED LIQUID DEVELOPER FOR DEVELOPING LATENT ELECTROSTATICIMAGES, SAID DEVELOPER CONSISTING ESSENTIALLY OF: (1) A TONER OF (A)FINELY-DIVIDED ELECTROSTATICALLY ATTRACTABLE SOLID OPAQUE PIGMENTPARTICLES SURROUNDED BY AN ENVELOPE OF (B) A RESINOUS BINDER THAT ISHIGHLY SOLUBLE IN AROMATIC SOLVENTS BUT NOT MORE THAN SLIGHTLY SOLUBLEIN ALIPHATIC SOLVENTS, SAID TONER BEING DISPERSED IN (2) AN ALKYL-ARYLSOLVENT SELECTED FROM THOSE REPRESENTED BY THE FORMULA: